Echoes characterized by a partial reflection of an incoming signal are produced in a transmission system whenever an impedance discontinuity or mismatch exists, such as at the junction between a four-wire transmission path and a two-wire transmission path. If the so-called echo-path delay is very small, then the reflected signal is typically not noticed at the source of the original signal. However, if the echo-path delay is otherwise, then the echo becomes, qualitatively, most annoying to the source, e.g., the talking party. One echo controlling device, known as an echo canceler, synthesizes an estimate of the echo signal and subtracts the estimate from an outgoing signal to effectively "cancel out" the echo signal. For example, FIG. 1 shows a transmission network, including a prior art echo canceler, coupling via hybrid 101 a bi-directional transmission path or facility 102 to a circuit arrangement including incoming unidirectional transmission path or facility 103 and outgoing unidirectional path or facility 104. Such an incoming and outgoing circuit arrangement constitutes, for example, a four-wire transmission facility.
Accordingly, incoming voice/speech signals are supplied via unidirectional path 103 to one input of adaptive filter 106-1 of canceler 106 and one input of NES (Near End Speech) detector 106-2. They are also supplied to hybrid 101 for presentation to station S1. (It is noted that incoming voice/speech signals received from the far end via path 103 will also be referred to herein as far-end speech and denoted by x(k), such signals are supplied to station S1 via hybrid 102. Similarly, outgoing voice/speech signals received from station S1 via bi-directional path 102, hybrid 101 and unidirectional path 104 are presented to an input of Near End Speech (NES) detector 106-2 and to one input (+) of adder circuit 106-3. (It is also noted that the voice/speech signals originating at station S1 will also be referred to herein as near-end speech (NES). The sum of NES and any echo will be denoted y(k), and the signals supplied to path 105 will be denoted by e(t).) NES detector 106-2 monitors path 104 in a conventional manner to distinguish echo from NES. If such monitoring and detection indicates that NES is present on path 104, then detector 106-2 outputs an adaptation control signal which inhibits filter 106-1 from adapting to the level of echo on path 104. If, on the other hand, no NES is detected, then the inhibit signal is removed to allow filter 106-1 to adapt, in a well-known way, to the echo signal on path 104. In doing so, filter 106-1 supplies to an input (-) of adder circuit 106-3 a signal which effectively constitutes an estimate of the echo signal. Adder circuit 106-3, in turn, subtracts the estimate from the signal received via path 104 and outputs the result to unidirectional path 105 for transmission to the far-end. It can be appreciated that filter 106-1 initially outputs a coarse estimate of the echo, but through its adaptation process eventually attains finer estimates of the path 104 echo. (It is noted that a similar echo cancellation process occurs at the far-end.)
As mentioned above, NES detector 106-2 attempts to distinguish between the echoed far-end speech (FES) and direct NES supplied via unidirectional path 104. It does this by comparing an estimate of the power level of the y(k) signal with an estimate of the power level of the x(k) signal. If the estimated level of y(k) is greater than some fraction (e.g., 0.5, which corresponds to -6 dB) of the level of x(k), then NES detector 106-2 concludes that the y(k) signal contains NES and inhibits the filter 106-1 adaptation process. However, if the comparison indicates that the power level of the x(k) signal exceeds that of the y(k) signal by the predetermined threshold, then detector 106-2 concludes that the y(k) signal is pure echo and allows the adaptation process to proceed.
The foregoing process for detecting for the presence of echo is imperfect at best. The reason for this is that measured short-term speech levels depend on who is talking and what they are saying at a particular instant of time. Moreover, transmission losses in the FES and NES transmission paths also affect such measurements. This is underscored by the fact that NES detection is less reliable in applications where transmission losses in the NES path exceed the transmission losses in the FES path. This "skew" is particularly apparent in cellular networks.